/*******************************************************************************
 * Copyright (c) 2011 MadRobot.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the GNU Lesser Public License v2.1
 * which accompanies this distribution, and is available at
 * http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 * 
 * Contributors:
 *  Elton Kent - initial API and implementation
 ******************************************************************************/
package com.madrobot.util;

import java.io.Serializable;
import java.util.Arrays;

/**
 * A <code>MultiKey</code> allows multiple map keys to be merged together.
 * <p>
 * The purpose of this class is to avoid the need to write code to handle maps
 * of maps. An example might be the need to lookup a filename by key and locale.
 * The typical solution might be nested maps. This class can be used instead by
 * creating an instance passing in the key and locale.
 * <p>
 * Example usage:
 * 
 * <pre>
 * // populate map with data mapping key+locale to localizedText
 * Map map = new HashMap();
 * MultiKey multiKey = new MultiKey(key, locale);
 * map.put(multiKey, localizedText);
 * 
 * // later retireve the localized text
 * MultiKey multiKey = new MultiKey(key, locale);
 * String localizedText = (String) map.get(multiKey);
 * </pre>
 */
public class MultiKey implements Serializable {
	// This class could implement List, but that would confuse it's purpose

	/** Serialisation version */
	private static final long serialVersionUID = 4465448607415788805L;

	/** The individual keys */
	private final Object[] keys;
	/** The cached hashCode */
	private final int hashCode;

	/**
	 * Constructor taking two keys.
	 * <p>
	 * The keys should be immutable If they are not then they must not be
	 * changed after adding to the MultiKey.
	 * 
	 * @param key1
	 *            the first key
	 * @param key2
	 *            the second key
	 */
	public MultiKey(Object key1, Object key2) {
		this(new Object[] { key1, key2 }, false);
	}

	/**
	 * Constructor taking three keys.
	 * <p>
	 * The keys should be immutable If they are not then they must not be
	 * changed after adding to the MultiKey.
	 * 
	 * @param key1
	 *            the first key
	 * @param key2
	 *            the second key
	 * @param key3
	 *            the third key
	 */
	public MultiKey(Object key1, Object key2, Object key3) {
		this(new Object[] { key1, key2, key3 }, false);
	}

	/**
	 * Constructor taking four keys.
	 * <p>
	 * The keys should be immutable If they are not then they must not be
	 * changed after adding to the MultiKey.
	 * 
	 * @param key1
	 *            the first key
	 * @param key2
	 *            the second key
	 * @param key3
	 *            the third key
	 * @param key4
	 *            the fourth key
	 */
	public MultiKey(Object key1, Object key2, Object key3, Object key4) {
		this(new Object[] { key1, key2, key3, key4 }, false);
	}

	/**
	 * Constructor taking five keys.
	 * <p>
	 * The keys should be immutable If they are not then they must not be
	 * changed after adding to the MultiKey.
	 * 
	 * @param key1
	 *            the first key
	 * @param key2
	 *            the second key
	 * @param key3
	 *            the third key
	 * @param key4
	 *            the fourth key
	 * @param key5
	 *            the fifth key
	 */
	public MultiKey(Object key1, Object key2, Object key3, Object key4, Object key5) {
		this(new Object[] { key1, key2, key3, key4, key5 }, false);
	}

	/**
	 * Constructor taking an array of keys which is cloned.
	 * <p>
	 * The keys should be immutable If they are not then they must not be
	 * changed after adding to the MultiKey.
	 * <p>
	 * This is equivalent to <code>new MultiKey(keys, true)</code>.
	 * 
	 * @param keys
	 *            the array of keys, not null
	 * @throws IllegalArgumentException
	 *             if the key array is null
	 */
	public MultiKey(Object[] keys) {
		this(keys, true);
	}

	/**
	 * Constructor taking an array of keys, optionally choosing whether to
	 * clone.
	 * <p>
	 * <b>If the array is not cloned, then it must not be modified.</b>
	 * <p>
	 * This method is public for performance reasons only, to avoid a clone. The
	 * hashcode is calculated once here in this method. Therefore, changing the
	 * array passed in would not change the hashcode but would change the equals
	 * method, which is a bug.
	 * <p>
	 * This is the only fully safe usage of this constructor, as the object
	 * array is never made available in a variable:
	 * 
	 * <pre>
	 * new MultiKey(new Object[] {...}, false);
	 * </pre>
	 * <p>
	 * The keys should be immutable If they are not then they must not be
	 * changed after adding to the MultiKey.
	 * 
	 * @param keys
	 *            the array of keys, not null
	 * @param makeClone
	 *            true to clone the array, false to assign it
	 * @throws IllegalArgumentException
	 *             if the key array is null
	 * @since Commons Collections 3.1
	 */
	public MultiKey(Object[] keys, boolean makeClone) {
		super();
		if(keys == null){
			throw new IllegalArgumentException("The array of keys must not be null");
		}
		if(makeClone){
			this.keys = keys.clone();
		} else{
			this.keys = keys;
		}

		int total = 0;
		for(int i = 0; i < keys.length; i++){
			if(keys[i] != null){
				total ^= keys[i].hashCode();
			}
		}
		hashCode = total;
	}

	// -----------------------------------------------------------------------
	/**
	 * Gets a clone of the array of keys.
	 * <p>
	 * The keys should be immutable If they are not then they must not be
	 * changed.
	 * 
	 * @return the individual keys
	 */
	public Object[] getKeys() {
		return keys.clone();
	}

	/**
	 * Gets the key at the specified index.
	 * <p>
	 * The key should be immutable. If it is not then it must not be changed.
	 * 
	 * @param index
	 *            the index to retrieve
	 * @return the key at the index
	 * @throws IndexOutOfBoundsException
	 *             if the index is invalid
	 * @since Commons Collections 3.1
	 */
	public Object getKey(int index) {
		return keys[index];
	}

	/**
	 * Gets the size of the list of keys.
	 * 
	 * @return the size of the list of keys
	 * @since Commons Collections 3.1
	 */
	public int size() {
		return keys.length;
	}

	// -----------------------------------------------------------------------
	/**
	 * Compares this object to another.
	 * <p>
	 * To be equal, the other object must be a <code>MultiKey</code> with the
	 * same number of keys which are also equal.
	 * 
	 * @param other
	 *            the other object to compare to
	 * @return true if equal
	 */
	@Override
	public boolean equals(Object other) {
		if(other == this){
			return true;
		}
		if(other instanceof MultiKey){
			MultiKey otherMulti = (MultiKey) other;
			return Arrays.equals(keys, otherMulti.keys);
		}
		return false;
	}

	/**
	 * Gets the combined hash code that is computed from all the keys.
	 * <p>
	 * This value is computed once and then cached, so elements should not
	 * change their hash codes once created (note that this is the same
	 * constraint that would be used if the individual keys elements were
	 * themselves {@link java.util.Map Map} keys.
	 * 
	 * @return the hash code
	 */
	@Override
	public int hashCode() {
		return hashCode;
	}

	/**
	 * Gets a debugging string version of the key.
	 * 
	 * @return a debugging string
	 */
	@Override
	public String toString() {
		return "MultiKey" + Arrays.asList(keys).toString();
	}

}
